142 CVA “HEFLAND 
a, DETERMINATION OF THICKNESS AND CHARACTER 
OF SURFACE FORMATIONS 
The chief potentialities of both the resistivity and the potential- 
drop-ratio methods lie in the determination of character and depth to 
formation boundaries of surface formations. In general, as in any other 
geophysical method, the thickness of geologic formations has to 
compare favorably with their depth, otherwise they may pass un- 
noticed in resistivity indications; on the other hand, conditions for 
recognizing formation boundaries are much more favorable in the 
potential-drop-ratio method than they are in the resistivity methods. 
In addition, a reason why these methods lend themselves so readily 
to the determinations of depths and thicknesses of surface formations 
is the fact that they often differ much more in their resistivity from 
the underlying series than these lower formations will differ between 
themselves. 
A problem which often arises in practical geology is the determina- 
tion of the thickness of glacial overburden. Both the :nethod of re- 
sistivity mapping and the method of vertical electrical drilling can 
be applied to advantage, depending on the problem. 
Thus far, the majority of examples for the determination of the 
thickness of surface formations have become known from applications 
in mining and civil engineering, but they would also be of importance 
in certain types of oil-structural work. 
I. BY EQUI-RESISTIVITY MAPPING 
A rather convenient way to map bedrock contours in a fairly 
large area, or to determine the depth to bedrock along a traverse 
line, is to carry a 4-terminal contacting arrangement with fixed elec- 
trode separation over the area and to observe and plot apparent re- 
sistivities in the manner shown in Figure 26. The results obtained with 
two electrode separations are illustrated. The solid contour represents 
the depth to bedrock, the dashed line is the indication obtained with 
the correct electrode separation, and the dotted lines show the ap- 
parent resistivities with too small electrode spacing. The geologic 
section was glacial drift above limestones and conglomerates. This 
figure shows that with an electrode separation larger than the greatest 
depth to bedrock, the bedrock contour may be mapped with a good 
deal of accuracy. 
2. BY ELECTRICAL VERTICAL DRILLING 
This method has been applied very frequently in late years in 
determining the depth to bedrock in tunnel and dam sites. Figure 27 
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